1. Field of the Invention
The present invention relates to thermoplastic polymers and a method of polymerization of crystalline thermoplastic polymers to high molecular weight polymers in the solid state. More particularly, it relates to a method of rapid solid state polymerization under conditions of mechanically induced surface stress and friction heat applied to polymer particles at incipient melt point temperatures of the crystalline polymers, followed immediately by quenching the polymers by direct contact evaporative cooling with cryogenic liquids, such as nitrogen. The rapid high temperature solid state polymerization provides high molecular weight polymers with superior mechanical properties and the immediate direct quench minimizes chemical degradation of the polymers and growth of large crystal agglomerates, spherulites, which cause polymer brittleness and degrade other mechanical properties of polymers.
2. Description of Prior Art
It is well known that low molecular weight polymers, or prepolymers, may be polymerized with, or without, catalysts, in the solid state to produce high molecular weight polymers. These polymers usually exhibit increased hardness and increased intrinsic viscosities. The solid state polymerizations of the prior art have usually been done by heating resin particles for long periods of,time at temperatures exceeding the glass transition temperatures but considerably lower than the resin melt temperatures.
During prolonged solid state heating polymer chains aggregate into supermolecular structures known as spherulites which are in some ways similar to grain structures in metals. Spherulites grow radially from a point of nucleation until other spherulites are encountered. Large spherulites cause brittleness and reduction in tensile strengths of polymers. The size of the spherulites can be controlled by the number of nuclei present with more crystalline nuclei resulting in more but smaller spherulites. Short heat times also result in smaller spherulites. The absence of large spherulites results in stronger less brittle polymers.
Duh in U.S. Pat. No. 5,750,644 provides a process for the crystallization and solid state polymerization of polyethylene naphthalate (PEN) at temperatures of about 20xc2x0 C. above the glass transition temperature (Tg) and about 10xc2x0 C. below the melting temperature (Tm) under special conditions for PEN only where pressure during the crystallization is higher than the vapor pressure of the volatile components in the PEN prepolymer. Duh states that the solid state polymerization time will normally range from 1 to 36 hours and preferably from 6 to 24 hours.
Duh in U.S. Pat. No. 6,403,762 disclosed a one step method of crystallizing and preheating polytrimethylene terephthalate (PTT) prepolymer without a drying/annealing step in between. The prepolymer still retains substantial moisture content when it is first exposed to the solid state polymerization temperature.
Ormand et al in U.S. Pat. No. 5,864,006 teach the preparation of high polymers of polycarbonates by heating a polycarbonate prepolymer with solvent and diluent, the prepolymer exhibiting a molecular weight of from 1,000 to about 20,000 under special conditions, at a temperature between about the prepolymer glass transition temperature less 50xc2x0 C. and 350xc2x0 C. A shearing force is applied at a rate of about 1 to 10,000 cycles per second. The shearing force is applied by a Haake Mixer or an extruder where the polycarbonate is melted.
Dujari et al in U.S. Pat. No. 5,955,569 disclose a method for solid phase polymerization of polyamide polymer using phosphorus catalysts in combination with an oxygen-free gas exhibiting very low Dew Points.
Solid state polymerization is made possible by an ability of a polymer to sustain a solid state at temperatures above the glass transition temperature without causing the polymer to fuse. When temperatures are held below the glass transition temperature polymerization cannot take place because the molecular movement is inhibited. Solid state polymerization takes place most rapidly at temperatures near to the melting temperature. It is difficult to operate at temperatures near to the melting temperature where heat is transferred into the polymer granules from external surfaces or gases without some of the particles reaching the melting temperature causing clumping and plugging of the solid state polymerization reactor.
In my U.S. Pat. No. 6,479,625B1 it was disclosed that the deformation of crystallizable thermoplastic polymer particles applied in a mechanically fluidized crystallizer creates heat of friction and mechanical stress on the particles to align the polymer molecules into crystallized polymers without agglomeration of the polymer.
Although the information provided in the foregoing references have advanced the art of polymerization of specific resins in the solid state, they have provided no solid state polymerization method applicable to thermoplastic resins which allows rapid and accurate polymerization to desired high intrinsic viscosities without formation of brittleness causing large spherulite crystals and chemical degradation of the polymeric chemicals.
It is therefore an object of this invention to provide a method of solid state thermoplastic polymer polymerization which provides rapid and accurate solid state polymerization to form mechanically superior polymers exhibiting high intrinsic viscosities without chemical degradation of the resin.
It is another object of this invention to provide a method of solid state polymerization wherein the friction between the polymer particles in a mechanically fluidized solid state polymerization reactor evenly generates heat sufficient to rapidly and evenly polymerize the polymers to the desired intrinsic viscosity.
It is a further object of the invention to provide a method which instantly stops the solid state polymerization reactions and the agglomeration of crystals to form large spherulites which cause polymer brittleness and reduce tensile strengths.
It is a still further object of the invention to provide a high strength thermoplastic polymer composition exhibiting high intrinsic viscosities which is free of large spherulites, produced by rapid solid state polymerization of crystallized polymer with heat created by friction between the resin particles in a mechanically fluidized polymerization reactor, with the particles of polymer instantly cooled to ambient temperature when the desired polymerization is achieved and before large spherulites are formed and chemical polymer degradation occurs.
I have now discovered that rapid solid state polymerization of crystalline thermoplastic polymers may be done effectively under conditions of mechanically induced surface stress and friction applied to polymer particles at incipient melt point temperatures of the crystalline polymers, followed immediately by quenching the polymers to temperatures below the glass transition temperatures of the polymers by direct contact evaporative cooling with cryogenic liquids. The new method of rapid high temperature solid state polymerization provides the preparation of high molecular weight polymers exhibiting high intrinsic viscosities, and superior mechanical properties and substantially eliminates chemical degradation of the polymers. The immediate direct quench by evaporating liquefied cryogenic gases directly on the polymer particle surfaces precludes the growth of large crystal agglomerates, spherulites, which cause polymer brittleness and degrade mechanical properties of the solid state polymerized polymers This discovery overcomes the limitations of the methods of the prior art and provides the objects of the invention with a practical method of solid state polymerization of crystalline thermoplastic polymers to form improved mechanical properties without chemical degradation.